[otp field] Prevent locked hidden autofill validation

[lunaxislu]

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Fixes #4828.

Follow-up to #4806 and #4810.

#4806 added a locked-state guard to OTP Field's hidden autofill input, but when the component is wrapped in Field.Root, the merged Field validation onChange can still run after the OTP Field handler returns. This can clear existing Form errors or commit validation while the field is readOnly or disabled.

This PR calls event.preventBaseUIHandler?.() before returning from the locked hidden-input autofill path, matching the Select, Combobox, and NumberField handling from #4810.

The tests now cover the locked-state matrix for hidden-input autofill:

• readOnly / disabled
• inside Field.Root / outside Field.Root

The outside-Field cases keep asserting that OTP Field public callbacks are not called. The inside-Field cases additionally assert that existing Field.Error content is preserved.

Verified with:

• pnpm test:jsdom OTPField --no-watch
• pnpm typescript

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4827
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4827
  

commit: 5095d90

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+28B(+0.01%)	🔺+2B(0.00%)

Details of bundle changes

Performance

Total duration: 1,336.12 ms +177.73 ms^(+15.3%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 2,050.49 ms +272.10 ms^(+15.3%)

Test	Duration	Renders
Select mount (200 instances)	186.46 ms 🔺+40.25 ms(+27.5%)	3 (+0)
Menu mount (300 instances)	168.81 ms 🔺+32.13 ms(+23.5%)	2 (+0)
Mixed surface mount (app-like density)	92.10 ms 🔺+25.78 ms(+38.9%)	5 (+0)
Scroll Area mount (300 instances)	97.40 ms 🔺+16.51 ms(+20.4%)	3 (+0)
Select open (500 options)	59.96 ms 🔺+15.23 ms(+34.0%)	14 (+0)

…and 1 more (+6 within noise) — details

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[atomiks]

Thanks for the fix @lunaxislu